PROJECT SUMMARY Christianson syndrome (CS) is a monogenic, X-linked neurologic disorder caused by changes in the Na+/H+ exchanger 6 (NHE6) that provides an opportunity to study both neurodevelopment and neurodegeneration (Gilfillan et al 2008). It is one of the most common X-linked disorders (Tarpey et al. 2009) and is notable for cerebellar pathology such as Purkinje cell loss (Garbern et al. 2010, Christianson et al. 1999) and cerebellar atrophy (Pescosolido et al. 2014). How NHE6 mutations cause cerebellar-associated pathology is unknown. By identifying the cellular mechanism underlying abnormal cerebellar development, novel therapeutic agents targeting progressive cerebellar symptoms can be tested. Our preliminary evidence recapitulates progressive Purkinje cell loss in NHE6 mutant mice, and motor dysfunction has been identified in these mice (Stromme et al. 2011, Sikora et al. 2015). Our previous research has shown loss of NHE6 results in endosomal overacidification, which leads to decreased neuronal arborization and BDNF signaling (Ouyang et al. 2013). Furthermore, a CS mouse model has shown to have deficits in endosomal-lysosomal functioning (Stromme et al. 2011), suggesting autophagy dysfunction as a potential mechanism in NHE6 mutations. Purkinje cell degeneration can be caused by autophagy deficits (Hara et al. 2006). Therefore, we hypothesize that NHE6 mutations cause endosomal overacidification and enhanced autophagy resulting in Purkinje cell that leads to abnormal cerebellar development and degeneration. We will also test the hypothesis these defects may be reversed by IGF-1 treatment in vitro and in vivo in our mouse model. We will address these hypotheses through the following Specific Aims. In the first aim we will demonstrate that an over-abundance of low pH endo-membranes in NHE6 mutant Purkinje cells is associated with abnormal Purkinje cell development that can be rescued or worsened by drugs in vitro. In the second aim we will elucidate a mechanism involving enhanced autophagy and decreased mTOR activity in NHE6-null Purkinje cells in vitro and in vivo that may be reversed by IGF-1 treatment. We will also examine whether IGF-1 treatment may ameliorate motor coordination deficits in NHE6 mutant mice. We will utilize cerebellar slices from an NHE6 null male mouse model to examine endo-lysosomal membranes, Purkinje cell development, autophagy, mTOR activity and IGF treatment. The information obtained from this project will provide a mechanistic understanding of cerebellar pathology in Christianson syndrome and, possibly, other neurologic disorders with cerebellar-related symptoms. Furthermore, this research may identify an FDA-approved drug, IGF, as a potential therapeutic for neurologic disorders with cerebellar dysfunction.